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springback using the finite element method (FEM) is still problematically, because the implemented material and
friction laws have to be optimized.
The assessment of the stamping part geometry using tactual coordinate measuring machines is state-of-the-art. This
time-consuming method is not capable e.g. to detect part shape fluctuations caused by spreading mechanical properties
of the sheet material. Contactless optical measurement techniques have advantages especially concerning the measuring
speed as well as the measuring area and therefore have the potential to be applied in-process.
Here projected light can be used to reproduce the stamping part geometry. In this case the reflection behavior of the
sheet surface has to be taken into account, especially on uncoated materials. An advantage of the grid projection is that
the part could be used in the further production process. However, grid projection is not suitable for the strain analysis.
3 VISIOPLASTIC ANALYSIS
For the design of new forming tools the knowledge of critical strain conditions in the stamping part is necessary to
avoid failures, e.g. cracks due to excessive sheet thinning. During the design of the forming tools numerical simulation,
especially finite element analysis (FEM) is used for the prediction of formability. During the try-out process and in case
of modifications of process parameters of the forming process (e.g. change of the sheet material) the experimental strain
analysis using the method of visioplasticity has to be applied. This method permits a statement about the influence of
the used sheet material, tool design, the blank shape and the tribological conditions on the final part quality.
Strain analysis in sheet metal forming is usually accomplished by marking a grid of known dimension on the flat sheet
blank and measuring the deformed grid on the part after the stamping process. For the evaluation of the strain-rates and
strain-distribution circles or square texture patterns have to be fixed to the surface to identify corresponding points at
different deformation stages. Following two different methods are described /9/, which have specific advantages and
disadvantages concerning an automated computer-based evaluation.
3.1 Circle grid analysis
In a homogenous forming process circles marked on the undeformed sheet will distort to ellipses of major and minor
axes, as illustrated in Figure 5. Assuming proportional deformation the principal directions are coincident with these
axes. Assuming incompressibility the principal strains €, €», 3 can be calculated with the principal elongations 1;, I,
and the circle diameter do:
1
=f] (1) sms Q) q,--(o, *9,) GB)
In sheet metal forming usually the forming limit diagram (FLD) is plotted as shown in Figure 6. The FLD provides
information about how much a particular metal can be stretched before necking or fracture occurs. The necking strains
are obtained under a variety of biaxial forming conditions so that most of the practical stamping conditions are
duplicated. Therefore, the FLD provides an indication of the material behavior under actual forming conditions. There
are several forming conditions of particular interest: stretch forming, plane strain, uniaxial tension and deep drawing.
The forming limit curve (FLC), also shown below, represents the material specific strain limit, which a part can bear
without fractures. It is plotted on the forming limit diagram to show how close a particular part is to the forming limit
under the specific strain state. The circle grid analysis is advantageous for finding strains at single measurement points
in sheet metal components by measuring the length and direction of the principal axes of the ellipse. However, it is less
convenient for a computer-based determination of strain distributions in whole regions.
(07-90; Qq,7-29, ¢,=0 9,799,
a forming limit
circle ellipse curve (FLC) -
l
plane strain
major strain p,
P
M
nO
Y
minor strain q,
Figure 5: Deformation of a circle in a homogenous field Figure 6: Forming limit diagram (FLD)
International Archives of Photogrammetry and Remote Sensing. Vol. XXXIII, Part B5. Amsterdam 2000. 293